1. Field of the Invention
This invention relates to a slip recovery system for a wound rotor motor.
2. Description of the Prior Art
In speed control drives for wound rotor induction motors, the slip-frequency power is recovered by the use of additional rotating machines or by converters. These drives are classically categorized as being either of the constant horsepower or constant torque type, the identification having reference to the inherent limitation in power based on full current and flux in the main machine. In the constant horsepower drive, the auxiliary machine is mounted on or mechanically coupled to the shaft of the main machine. The slip energy is converted into mechanical energy and returned to the main motor shaft by the auxiliary machine. In the constant torque system, the slip energy is converted into electrical energy of the frequency and voltage of the voltage supply, and is returned or fed back to the supply. Since this power is not delivered to the main motor shaft, the auxiliary machine is not mechanically coupled to the motor shaft but is separately driven. The limiting torque of the main motor being constant, the maximum horsepower output is proportional to the operating speed.
The emergence of silicon controlled rectifiers (SCR's) with increasingly higher current ratings, has given impetus to solid-state packaged drives for polyphase motors. In the solid-state version of the slip energy recovery system, the slip ring power is rectified in a diode bridge, and then passed over a DC link to a supply line commutated inverter that returns the slip power to the voltage supply. The speed of the main motor is controlled by the firing angle of the SCR's which comprise the inverter. Slip recovery systems of this type are economical for narrow speed ranges such as for fans and pumps or where the horsepower rating of the induction motor is so large that the costs of the controls are therefore of minimal significance.